In recent years, motor vehicles have been often fitted with a pretensioner device which positively increases the tension of the seat belt for restraining the vehicle occupant at the time of a crash and improves the protection of the vehicle occupant. The deceleration acting on the vehicle occupant who is restrained to the seat by a restraint device such as a seat belt starts rising only when the forward inertia force acting on the vehicle occupant at the time of the crash has started to be supported by the seat belt. As it is not possible to eliminate a certain amount of resiliency and slack in the seat belt, the deceleration of the vehicle occupant reaches a peak level only when the vehicle occupant has moved forward a certain distance under the inertia force and the elongation of the seat belt has reached its maximum extent. The peak value of the deceleration of the vehicle occupant gets greater as the forward displacement of the vehicle occupant under the inertia force increases, and is known to be substantially larger than the average deceleration of the passenger compartment of the vehicle body.
When the relationship between the vehicle body deceleration and the vehicle occupant deceleration is compared to the relationship between the input and output of a system consisting of a spring (vehicle occupant restraint device) and a mass (mass of the vehicle occupant), it can be readily understood that the maximum elongation and time history of the spring are dictated by the waveform (time history) of the vehicle body deceleration. Therefore, it can be concluded that the waveform of the vehicle body deceleration should be controlled in such a manner that not only the average deceleration acting on the vehicle body is reduced but also the overshoot of the vehicle occupant deceleration due to the elongation of the spring (vehicle occupant restraint device) is minimized.
In the conventional vehicle body structure, the impact energy is absorbed by a crushable zone, consisting of an impact reaction generating member such as side beams and gaps defined between various components, provided in a front part of the vehicle body, and the waveform of the vehicle body deceleration is adjusted by changing the resulting reaction properties by means of the selection of the dimensions and deformation properties of such parts. The deformation mode of the vehicle body other than the passenger compartment at the time of a crash may also be appropriately selected so that the deceleration of the passenger compartment of the vehicle body may be reduced, and the deformation may be prevented from reaching the passenger compartment. Such vehicle body structures are proposed in Japanese patent laid open publication (kokai) No. 07-101354.
It is important to note that the injury to the vehicle occupant at the time of a vehicle crash can be minimized by reducing the maximum value of the acceleration (deceleration) acting on the vehicle occupant which is dictated by the waveform (time history) of the vehicle body deceleration. It is also important to note that the total amount of deceleration (time integration of deceleration) which the vehicle occupant experiences during a vehicle crash is fixed for the given intensity of crash (or vehicle speed immediately before the crash). Therefore, as shown in FIG. 6 for instance, the ideal waveform (time history) of the vehicle body (seat) deceleration (G2) for the minimization of the vehicle occupant deceleration (G1) should consist of an initial interval (a) for producing a large deceleration upon detection of a crash, an intermediate interval (b) for producing an opposite deceleration, and a final interval (c) for producing an average deceleration.
The initial interval allows the vehicle occupant to experience the deceleration from an early stage so that the deceleration may be spread over an extended period of time, and the peak value of the deceleration to be reduced. According to a normal vehicle body structure, owing to the presence of a crushable zone in a front part of the vehicle and a slack and elongation of the restraint system such as a seat belt, it takes a certain amount of time for the impact of a crash to reach the vehicle occupant. The delay in the transmission of deceleration to the vehicle occupant must be made up for by a subsequent sharp rise in deceleration according to the conventional arrangement. The final interval corresponds to a state called a ride-down state in which the vehicle occupant moves with the vehicle body as a single body. The intermediate interval is a transitional interval for smoothly connecting the initial interval and final interval without involving any substantial peak or dip in the deceleration. Computer simulations have verified that such a waveform for the vehicle body deceleration results in a smaller vehicle occupant deceleration than the case of a constant deceleration (rectangular waveform) for a given amount of deformation of the vehicle body (dynamic stroke).
According to the conventional vehicle body structure, the vehicle body components of the crushable zone start deforming from a part having a relatively small mechanical strength immediately after the crash, and a part thereof having a relatively high mechanical strength starts deforming thereafter. As a result, the waveform of the crash reaction or the vehicle body deceleration is small in an early phase, and then gets greater in a later phase so that the vehicle occupant deceleration cannot be adequately reduced. To eliminate such a problem, it has been proposed to obtain a prescribed amount of reaction force by making use of the collapsing of the side beams and to maintain a stable reaction by providing a plurality of partition walls in the side beams (Japanese patent laid-open publication (kokai) No. 07-101354). However, such previous proposals can only maintain the vehicle body deceleration at an approximately constant level at most, and are unable to provide a more effective deceleration waveform.
To minimize the adverse effect of the resiliency of the seat belt, it is known to provide a pretensioner device in association with the seat belt to positively tension the seat belt at the time of a vehicle crash. According to another previously proposed structure, at least one of the anchor points of the seat belt is attached to a member which undergoes a movement relative to the remaining part of the vehicle which tends to increase the tension of the seat belt in an early phase of a vehicle crash. Such devices are beneficial in reducing the maximum level of deceleration acting on the vehicle occupant at the time of a vehicle crash, but a device capable of more precise control of the vehicle occupant deceleration is desired.
Referring to FIG. 7, the vehicle occupant deceleration G1 and vehicle body deceleration G2 correspond to the input and output of a transfer function representing a two-mass spring-mass system consisting of the mass Mm of a vehicle occupant, a spring (such as a seat belt), and a vehicle body mass Mv. More specifically, the vehicle body deceleration G2 can be given as a second-order differentiation of the coordinate of the vehicle body mass Mv with respect to time.
However, in an actual automotive crash, if a three-point seat belt is used, the shoulder belt portion of the seat belt which can be considered as a spring engages the chest of the vehicle occupant corresponding to the center of the vehicle occupant mass Mm so that the shoulder belt portion can be considered as consisting of two springs, one extending between the chest and shoulder anchor, the other extending between the chest and the buckle anchor.
If the seat belt is entirely incorporated to the seat, the shoulder anchor and buckle anchor move as a single body, and the two parts experience an identical deceleration. In such a case, it can be assumed that the seat belt can be given as a composite of two springs, and the deceleration acting on the shoulder anchor and buckle anchor is identical to the input to the two-mass spring-mass system or the vehicle body deceleration.
Now, suppose if the buckle anchor point is fixedly attached to the vehicle body while the shoulder anchor is capable of movement relative to the vehicle body as an example in which the two anchor points undergo different movements relative to the vehicle body. In such a case, because the shoulder anchor and buckle anchor experience different decelerations, the springs cannot be simply combined or the decelerations acting on the shoulder anchor and buckle anchor cannot be simply equated to the vehicle body deceleration.
Meanwhile, the external force acting on the chest wholly consists of the force received from the seat belt. Therefore, if the time history of the load acting on the seat belt in the direction of deceleration agrees with the time history of the spring load in the two-mass spring-mass system, the chest receives the same deceleration waveform as the response of the vehicle occupant mass of the two-mass spring-mass system to the optimum waveform of vehicle body deceleration. This enables the vehicle occupant to reach the ride-down state in which the vehicle occupant is restrained by the seat belt substantially without any delay and the relative speed between the vehicle body and vehicle occupant is zero (no difference between the vehicle occupant deceleration G1 and vehicle body deceleration G2).
To achieve a time history of the seat belt that produces such a state, it suffices if the time history of the average deceleration of the shoulder anchor and buckle anchor (or vehicle body) is equal to the optimum waveform of the vehicle body deceleration. Introducing the concept of the waveform of average vehicle body deceleration allows an identical result in reducing the vehicle occupant deceleration as controlling the vehicle body deceleration so as to achieve the optimum waveform to be achieved.
The early rise in the tension of the seat belt to apply the deceleration to the vehicle occupant from an early stage can be most conveniently provided by a pyrotechnical actuator typically using a propellant. Pyrotechnical actuators are widely known in such applications as vehicle air bags and pretensioners. However, it was found due to the nature of its structure which relies on the generation of high pressure gas that such an actuator alone may not be able to produce a desired time history of the deceleration of the vehicle occupant. It was found that the provision of inertia mass prevents an oscillatory movement to the moveable end or vehicle occupant during the activation of the actuator. The inventors have discovered that such a problem can be overcome by adding a suitable amount of mass to the actuator end of the seat belt in combination with a cushioning member.